What does 98.8% purity on a C18 column actually tell me about a semaglutide vial?

Setup, so nobody has to ask: 98.8% · a C18 column · semaglutide.

I have read the primary source rather than the summary, which has left me with more questions.

I understand the headline. I do not understand the footnotes, and the footnotes look important.

What would I need in addition before this supported a decision?

It helps to be literal here: the chemistry of peptides at low pH and high organic-solvent concentration is not something most users have intuition for, which is why published methods exist.

Detector linearity is why a heavily loaded peak can give false purity by compressing the main peak height while leaving impurity shoulders unchanged.

System suitability checks on replicate injections of a known standard establish whether the method was in control — if the peak area varies by more than two per cent between replicates, something is wrong.

Peptide impurities from solid-phase synthesis include deletion sequences, truncations from premature cleavage, racemised residues from epimerisation and oxidised variants, each of which may have different chromatographic behaviour.

The limitation is that co-elution is invisible — if two species happen to have the same retention time, they will report as a single peak at their combined area.

The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.

In practice, the limit of this technique for these molecules is reaching a point where small improvements require proportionally large investments in method development.

Formic acid is the compromise when you need the mass spectrometer on the same run, but the peak shape penalty is real and easily a tenth of a per cent on purity.

Specifically, sample preparation is almost always under-appreciated — a reconstituted peptide in strong solvent will distort its own peak on the gradient.

The selectivity of a reverse-phase separation depends on the hydrophobicity of the side-chain profile, which is why two peptides can co-elute even if they differ by a residue.

If two labs give different numbers, the method difference is the first hypothesis, not lab quality.

edited 13 Aug 2024 by plate_count_9k — added a caveat about sampling

Worth being precise here: method transfer between laboratories is almost impossible without full documentation, which is why two labs reporting the same preparation should expect disagreement.

Reverse-phase stationary phases use C18 or C8 chains bonded to silica, and the pore size of the silica matters more for a peptide of this chain length than the ligand length does.

Inter-laboratory studies using identical methods on identical material show precision well within half a per cent when the method is locked down, pointing to method variability as the primary source of disagreement.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

edited 13 Aug 2024 by p_mkhize — corrected a unit error in the worked example

The relevant detail is that reverse-phase HPLC is the workhorse for peptide purity work, but it is almost universally run under conditions that are not optimal for a peptide of this chain length.

Wider-pore phases — 300 Angstrom rather than 100 Angstrom — have faster mass transfer and narrower peaks for peptides above three kilodaltons, which is almost every peptide you will see.

I would caution against over-interpreting small differences between runs, because the noise floor of the method is larger than most people assume.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.